1. Field of the Invention
The present invention relates to power converters and more specifically to high efficiency charge pump circuits.
2. Description of Related Art
Charge pump circuits are often required within integrated circuit's (IC's) to convert a low voltage to a high voltage. Typically, such charge pump circuits employ a series of clocked diode-capacitor voltage multiplier circuits, as shown in FIG. 1. Such charge pump circuits generate only a small amount of output current, typically less than 100 .mu.A, thereby limiting their driving ability. Increasing the size of the capacitors to provide a greater output current not only undesirably increases the size of the charge pump circuit but also may lead to increased noise.
The output voltage V.sub.PP of charge pump circuit 100 is a function of the input power supply, the number of pump stages, the clock frequency, and the load current at the output terminal. This relationship may be expressed as: ##EQU1## where V.sub.CC is the power supply, N is the number of pump stages, V.sub.PN is the forward voltage drop across each of diodes D.sub.1 -D.sub.N, I.sub.out is the load current, f.sub.e is the effective clock frequency, and C.sub.s is the parasitic capacitance at each of the clocked nodes along the diode chain.
In U.S. Pat. No. 5,216,588, Bajwa et al disclose a charge pump circuit which may increase output current to approximately 8 mA by providing a plurality of parallel-connected diode-capacitor voltage multiplier circuits. The plurality of voltage multiplier circuits are switched at different times during the clock frequency to minimize noise generation. Although improving performance over more conventional charge pump circuits, the greater output current is realized by the addition of parallel diode-capacitor voltage multipliers, thereby significantly increasing the size and complexity of the charge pump.
It would thus be advantageous for a charge pump circuit to provide a greater current driving capability without significantly increasing the size of the charge pump circuit.